For the past few decades, detectors for single photon detection have been based on photomultiplier tubes (PMTs) as the photodetector of choice. The drawbacks of PMTs typically include relatively high cost, large size and high sensitivity to magnetic fields, in addition to the limited spatial resolution and deadtime effects. To address these drawbacks, the possibility of using compact solid state photodetectors, such as silicon photomultipliers (SiPMs), has been explored. SiPMs have a similar gain to that of PMTs. In addition, SiPMs have a number of advantages over PMTs, such as compactness, low bias voltage operation, magnetic field insensitivity and fast timing response. SiPMs also take advantage of the highly developed Si process technologies and the modern fabrication facilities for batch-processing in semiconductor industry, which provide the robustness and low fabrication costs of the devices. SiPMs have therefore emerged as the photodetector of choice for widespread applications in high-energy physics, fluorescence and luminescence decay measurements, single-molecule detection, laser ranging, nuclear medical imaging like Positron Emission Tomography (PET), radiation detection for homeland security systems, and so on.
FIG. 1 shows a typical layout 100 for a conventional SiPM pixel 102. As can be seen in FIG. 1, hundreds or thousands of microcells 104 are arranged side by side. The cathodes of each microcell are connected together as provided by an electrode 108, and the anodes of each microcell are connected together as provided by an electrode 106. The two electrodes 106,108 shown in FIG. 1 may be connected with adjacent pixels or corresponding readout circuits (not shown in FIG. 1). Although the SiPM pixel 102 is relatively large in size, typically a few millimeters, it only gives one output regarding the light intensity detected in the pixel area. In order to obtain an image showing the light distribution, many SiPM pixels need to be tiled to form a SiPM array.
FIG. 2A shows an image 200 illustrating an example of a commercially available SiPM pixel 202. The pixel 202 is packaged based on surface-mount technology (SMT), so that it may be 4-side tileable. This way, a SiPM array 220 may be flexibly formed by tiles 204 of such SiPM pixels 202, as shown in an image 222 illustrated in FIG. 2B. However, since the SiPM pixel 202 is separately packaged, the area of the active region in a pixel 202 is considerably smaller than the area of a tile 204, because certain amount of the tile area is consumed by the edge 206 of the SMT package. When photons are launched to this edge-consumed area, these photons could not be detected. The ratio between the area of the active region in a SiPM array (the total area of all the microcells in all the pixels) and the area of the array is defined as tile fill factor. The larger the tile fill factor is, the higher the photon detection efficiency would be. Other than the SiPM array tiled by separately packaged SiPM pixels, a few SiPM pixels may also be packaged together to form a module of SiPM arrays.
An example of a 4×4 SiPM array 300 packaged by through-hole technology (THT) is shown a schematic view 302 in FIG. 3. As can be seen in FIG. 3, the sixteen SiPM pixel dies 304 are arranged on a printed circuit board (PCB) or a ceramic cavity with bonding pads 306. The terminals 308 of each pixel 304 are connected to pads 310 by wire-bonding. Since the SiPM pixels 304 are not separately packaged, the tile fill factor of the SiPM array 300 may be increased as compared to the array of FIG. 2B.
FIG. 4 shows an image 400 illustrating an example of a commercially available module of 4×4 SiPM array 402, which is based on the technology shown in FIG. 3. However, the SiPM pixels 304, 404 in FIG. 3 and FIG. 4 are unable to be closely arranged side by side. A gap 312, 406 with certain width is necessary between adjacent pixel dies in order to accommodate the bonding pads and facilitate wire-bonding operation. Thus, there is a need for a more compact tile scheme where the gaps between adjacent pixel dies become unnecessary and the dies may be closely tiled side by side in order to achieve higher tile fill factor and photon detection efficiency, thereby addressing at least the problems mentioned above.